1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method. The present invention also relates to a mask or reticle masking device for use in a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A prior art lithographic apparatus includes an illumination system configured to provide a beam of radiation, wherein the illumination system defines a focal plane through which, in use, the beam of radiation passes; a support configured to support a patterning device at a location, the patterning device configured to pattern the beam of radiation according to a desired pattern; a masking device for obscuring at least a part of the patterning device from the beam of radiation, the masking device including a first masking device arranged to obscure part of the location in a first direction with respect to the location, a second masking device arranged to obscure part of the location in a second different direction with respect to the location; a substrate table configured to hold a substrate; a projection system configured to project the patterned beam onto a target portion of the substrate.
The term “patterning device” should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” has also been used in this context. Generally the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include a mask. The concept of a mask is well known in lithography, and includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array is another example of a pattering device. The array is a matrix addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required addressing can be performed using suitable electronic means.
Another example of a pattering device is a programmable LCD array. As above, the support in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereinabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the “projection lens” or “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types to direct, shape or control the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory processes may be carried out on one or more tables while one or more other tables are being used for exposures.
Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (including one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In prior art apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the beam of radiation in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned.
It is often desirable or necessary to ensure that only a certain part of the mask is imaged by the beam of radiation to the substrate. For example, the mask may contain more than one pattern of which only one is used for a given exposure. It is also often desirable or necessary to stop stray light from impinging on the substrate. In lithographic projection systems using this function such is typically achieved by providing a reticle masking device at an intermediate plane in the illuminator.
There are currently two types of reticle masking schemes: masking for static exposure in a stepper system and masking for scanning exposure in a step-and-scan apparatus. In a stepper system where the mask is fixed with respect to the illuminator, the reticle masking device is provided adjacent to the mask and is also fixed with respect to the illuminator and the mask. In static exposure, part of the mask is blocked from the illumination beam for the duration of an exposure. In a step-and-scan system where the mask is movable with respect to the illuminator, the reticle masking device is provided adjacent to the mask and is also movable with respect to the illuminator and the mask. In scanning exposure, a part of the mask is blocked for a predetermined length of time.
A reticle masking device may include at least one movable blade. In certain devices two sets of moveable blades are provided. Typically the two sets of blades are mechanically coupled to a support frame and each support is mounted on a common frame. Thus the sets of blades are mechanically coupled to each other.
FIGS. 17(a) and 17(b) depict a conventional masking device MD, which may be used in a lithographic apparatus. The masking device MD is disposed in an XY plane, X being substantially perpendicular to Y and the XY plane being substantially perpendicular to the Z-direction. The masking device MD includes four movable blades arranged in two pairs X1 and X2, disposed along the X axis, and Y1 and Y2 disposed along the Y axis. As shown in plan view in FIG. 17(a), each blade forms an L-shape, with a rectangular portion disposed close to the Z-axis and an arm which extends away from the Z-axis so that the blades may be connected to suitable actuators. Blades X1 and X2 are movable in the X-direction, and blades Y1 and Y2 are moveable in the Y-direction.
The cross-sectional view in FIG. 17(b) shows how the blades are arranged in the Z-direction to allow them to overlap. Note that in practice the blades are made very thin in the Z-direction, and the X and Y blades are very close together in Z such that the four blades form a rectangular slit SL lying in an optical position along the Z-direction.
FIG. 17(b) also shows an embodiment of the masking device lens MDL, including two lenses LS1 and LS2, where a lens may be a single optical element or a group of optical elements.
In step mode, the position and size of the masking slit (SL, FIG. 17(a)) is chosen such that the beam of radiation PB will be incident upon the required circuit pattern of the patterning device MA. The masking device MD is kept relatively stationary and the entire circuit pattern is projected onto the target portion C in a single static exposure.
In scan mode, the masking device MD is configured to form a masking slit SL having a larger dimension in the X-direction than the Y-direction. For scanning exposures, Y1 and Y2-blades are arranged to be movable during the exposure, and X1 and X2-blades, although movable, are generally arranged to be stationary during the exposure. For scanning exposures, the Y1 and Y2-blades in particular are arranged to perform additional movements to allow scanning of the patterning device MA with respect to the beam of radiation PB.
Before a scanning cycle starts, the Y-blades are arranged to prevent any radiation impinging on the patterning device MA. At the beginning of the scanning cycle, the Y-blades open to a scanning distance. At the end of the scanning cycle, the Y-blades move into a position in which light is prevented from impinging on the patterning device MA.
In case a different portion of the patterning device needs to be masked, the X-blades are generally moved between consecutive exposures.
With conventional masking devices, as higher scanning speeds are demanded, conventional masking devices fail. In particular, with some conventional masking devices the mass of the coupled blades creates inertia, preventing the scanning blades from being able to be accelerated and decelerated fast enough to open to their scanning position, and close at the end of the scan position, respectively. Further, the high moving mass cannot be satisfactorily moved at high enough scanning speeds by conventional motors without causing disturbances to be transferred to other parts of the apparatus.
Additionally, at the start and end of the scans the X- and Y-blades, collectively referred to as reticle masking blades, respectively open and close an illumination slit. For this purpose, two scanning blades are used in the Y-direction. When blocking radiation the blades must absorb a substantial amount of energy from an illumination source, e.g. a laser source. This gives rise to thermal problems. Further, two Y-blades must be moved requiring two motors, amplifiers, etc.
Motors for driving the blades are controlled by software which includes a list of instructions to control the motors. In conventional reticle masking devices it is possible for blades to collide with one another in the case of a malfunction. This may not only damage the blades, but will affect production if the apparatus is shut down for repairs.